The field of the invention is in particular, but not exclusively, that of telescope mirrors and more particularly of space telescope mirrors. In recent years, new concepts have been developed making it possible to produce new optical functions. Mention will be made, for example, of the deployment of lighter telescopes in space, the production of large-diameter telescopes based on segmented mirrors, the installation of twinned telescopes allowing the synthesis of aperture or the production of large interferometers.
Naturally, the optical aberrations and defects of these telescopes must be perfectly known and controlled so as to maintain a perfect image quality. In particular, the astigmatism of the mirrors and/or the radius of curvature must be known and corrected. In a simple manner, a mirror is called astigmatic when, in two orthogonal sectional planes passing through its centre, it has two different radii of curvature. There are various techniques making it possible to control the astigmatism. It is possible to use, for example, an active control device making it possible to modify the shape of a plurality of secondary mirrors forming a main mirror. These devices are not qualified for space applications, are very heavy, their reliability is limited and their MTBF (mean time before failure) is relatively short.